1. Field of the Invention
The present invention relates to a liquid container in which a porous member is installed, a fuel cell system using the liquid container, and a method for controlling the fuel cell system.
2. Description of the Related Art
Various effects can be obtained by installing a hydrophilic porous member within a fuel container or a liquid container, such as an ink container for an ink-jet printer, having a form similar to that of a fuel container. Specifically, by installing a hydrophilic porous member, the liquid can be sucked out from the container no matter which direction the container faces with respect to the direction of the gravity.
However, the installation of the hydrophilic porous member reduces, by the amount of its own volume, the volume of the liquid accommodated in the container. Accordingly, it is necessary to suck out as much fuel as possible from the hydrophilic porous member. However, suppose the case where the liquid is sucked from a container through a hydrophilic porous member with a pump. In this case, when the residual amount of the liquid contained in the hydrophilic porous member becomes not more than a specific amount, air bubbles will enter the sucked liquid, and consequently the liquid with air bubbles will directly flow into a liquid pump to adversely affect the pump performance.
Accordingly, it is required to detect the residual amount of the liquid contained in the hydrophilic porous member at the moment when air bubbles start to enter the liquid which is being sucked with a pump (hereinafter, such a residual amount referred to as “near end”). In addition, it is required to prevent air bubbles from entering the fuel that is sucked from the fuel container upon detection of the near end.
There is, for example, the following method for preventing the entry of air bubbles. In this method, a cavity part is provided between a hydrophilic porous member A and a hydrophilic porous member B that is disposed at a position closer to a suction port than the hydrophilic porous member A. Then, the near end is detected by visually checking whether or not an air bubble enters this cavity part. As can be seen in the configuration of a water-based-ink pen or the like, the liquid suction capability of the hydrophilic porous member B is, in many cases, set higher than that of the hydrophilic porous member A for the purpose of increasing the suction rate of the liquid.
However, with this near end detection, air bubbles will eventually start to enter the sucked liquid unless the container is promptly replaced before the liquid in the cavity part is depleted. A time sufficient for the replacement of a container can be obtained if the suction of liquid is stopped completely once. However, in a case where this method is applied to a fuel cell system or the like, it is not suitable to completely stop the suction of liquid in view of the operation performance. Moreover, although a time sufficient for the replacement of the container can be obtained by providing a cavity part with a sufficient size, the area of a window necessary for the visual check will increase. Even if the air bubble detection is performed by using an optical or electrical mechanism instead of the visual check, the detection area will increase as the window becomes large. Therefore, for miniaturizing the liquid container or the liquid consuming apparatus, it is disadvantageous to provide a cavity part with a sufficient size and is thus not suitable, either.
In an example shown in JP-A H5-42680 (KOKAI), a part of the wall of an ink tank, which is in contact with a porous member, is formed of an acrylic resin, and a plurality of groove parts different in capillary force are formed in the inner surface of the acrylic wall. Here, the residual amount of ink can be detected by utilizing the fact that the state of an ink entering the grooves formed as capillary tubes changes due to the magnitude relation between the capillary force of the porous member and that of the groove on the wall of the ink tank. Installing the above-described mechanism in the suction port of the liquid container makes it possible to detect the near end.
However, particularly in the case where the ink is sucked with a pump, at the time when air bubbles enter the groove part and the near end is detected, a lot of air bubbles have already entered the porous member around the groove part. Accordingly, air bubbles can enter the sucked ink as well.
An example shown in U.S. Pat. No. 6,431,672 includes ink reservoirs different in capillary forces. The ink reservoir having the higher capillary force is provided with an ink outlet and an ink level sensor. The ink level sensor is a C-shaped tube with both ends connected to the ink reservoir having the higher capillary force. Here, the capillary force is designed so that the ink in the tube is depleted when the amount of ink in the ink reservoir having the higher capillary force becomes low, thereby obtaining the function to detect the near end.
However, particularly in the case where the ink is sucked with a pump, a large number of air bubbles have already entered the ink reservoir having the higher capillary force when air bubbles enter the ink level sensor and the near end is detected. Accordingly, air bubbles can enter the sucked ink as well.
In addition, if the ink level sensor is connected not to the ink reservoir having the higher capillary force but to the ink reservoir having the smaller capillary force, the ink reservoir tank having the higher capillary force may achieve a state where there is almost no entry of air bubbles when the ink level sensor detects the near end of the ink reservoir.
However, this near end detection will detect a state where the residual amount of ink is more than that in the case where the ink level sensor is connected to the ink reservoir having the higher capillary force. Therefore, in order to detect a state where the residual amount of ink is as small as possible, a more creative study is required.